Rail operators require both mainline locomotives for train haulage and shunt locomotives for shunting functions. Remote control locomotives which can be controlled away from the vehicle are advantageous for railyard shunting and are usually specially fitted out for remote operation. Manually controlled and remote control locomotives are often dedicated to their respective functions and cannot be interchanged without refitting.
U.S. Pat. No. 4,687,258, discloses a portable remote control system which includes a receiver unit which plugs into a locomotive's train line socket to control train line circuits. Some mainline locomotives are equipped with a "train line" (usually an electrical control signal interface) and brake control line (usually pneumatic airline interface) which enable one locomotive to be linked with another locomotive for multiple operation. The portable remote control system also includes pneumatic equipment for connection into the locomotive's pneumatic braking systems. The system bleeds air from the train air brake line to control the independent locomotive brakes. The bleed of air is limited so the small loss of air from the train air brake line does not apply the train brakes (which are normally operated by a reduction in air pressure). A transmitter unit remote from the locomotive allows an operator to send commands or control signals to the receiver unit which are then implemented by control of the train line circuits and braking systems. The system is dedicated to one specific type of train line socket and train line circuit arrangement as received commands are implemented directly by energising a corresponding relay. The system also acts as a slave to the transmitter commands, implementing the commands irrespective of whether they represent "safe" or otherwise allowable commands.